The flat-plate solar collector is the essential piece of equipment used to convert solar radiant energy into a heated fluid. Many configurations of flat-plate collectors have been designed and used. In parts of Australia, Israel and Japan flat-plate collectors are the standard method of providing domestic hot water.
The operation of a flat-plate collector is inherently transient. The driving forces of solar radiation, wind, ambient temperature and weather are continually changing. There is no steady state or equilibrium condition. Therefore, existing flat-plate solar collectors are designed to operate best at some average condition, usually with degraded performance when the solar radiation is at its maximum.
The efficiency of a flat-plate collector is defined as the percent of solar radiation striking the collector which is converted to useful energy. This efficiency drops sharply with a rise in collector temperature. The lower the temperature, the higher the efficiency of collection of solar energy. Collector losses are always minimized when operating at the lowest mean collector temperature. Existing collectors have highest losses when solar radiation is at its maximum.
As the mass flow rate through a collector increases, the temperature rise through the collector decreases. This decrease under high heat solar conditions contributes to good collector performance. Uniform flow rates throughout a collector are also important in obtaining good performance. If the flow is not uniform, parts of the collector will run at significantly higher temperatures with degraded performance. For maximum useful energy gain, the average collector temperature must be minimized. Higher collector temperatures always result in poorer collector performance.
The reliability of flat-plate collectors has been reduced by failures due to pressure surges, freezing and corrosion. Even the earliest sheet-and-tube flat-plate collectors experienced these types of failures; the new configurations in which formed sheets are joined to provide flow passages appear to have an equally high incidence of rupturing.
Noises generated in solar collectors, radiators and heat exchangers are especially bothersome because of the almost continuous pumping of fluids in a solar system used for heating and cooling. For example, in a heating mode or cycle, fluids are pumped through collectors whenever the sun is out. In a cooling mode or cycle, fluids are pumped through collectors at night. In addition, the hot or cold fluids are pumped to radiators or heat exchangers whenever a thermostat signals for higher or lower temperatures in a room.
Some of the foregoing problems might be overcome by providing a separate heat-sensing flow-control valve added to a flat plate collector to increase its efficiency. Such would be possible if the flow channels in the collector were sufficiently large. With such an arrangement, two devices would be needed: one for heating and a different one for cooling. Such a system could become cumbersome as well as expensive.